1. Field of the Invention
The present invention relates to centrifugal clutches for motorcycles, and a method and apparatus for converting a stock manufacturer manual clutch into a centrifugal clutch for a motorcycle.
2. Description of the Related Art
A typical motorcycle clutch includes a clutch basket, a hub, a cover and clutch plates. A typical motorcycle has two types of plates, a steel plate and a fiber plate (the fiber plate is usually constructed of an aluminum substrate with a fiber material the substrate). A typical motorcycle clutch has eight fiber plates and seven steel plates structured in an alternating manner with a steel plate positioned between each of the fiber plates. The fiber plates typically have twelve tabs that are positioned between fingers of a clutch basket which allow the fiber plates to move with the clutch basket. Each of the steel plates has interior teeth that fit into a spline of an aluminum hub allowing the steel plates to move with the aluminum hub. Each of the steel plates preferably has a thickness of approximately 1.5 millimeters (“mm”) and each of the fiber plates typically has a thickness of 3 mm. The steel plates are 1.5 mm thick because the teeth are forced against the aluminum hub, which can groove quite easily due to the massive amount of force. The exception to this general rule is a KTM rfs motor (no longer in production) which used 1 mm steel plates because the steel plates ran against a steel hub.
The clutch basket is connected to a crankshaft, and the hub is connected to the transmission. The only connection between the engine and gear box is the friction caused by the engagement of the fiber plates against the steel plates, and the engagement comes from the force exerted by multiple coil springs. The coil springs hold the engine and transmission together at all times except when the clutch lever is pulled by a rider. When the clutch lever is pulled, a cam or slave is actuated which compresses the coil springs thereby creating gaps between the fiber plates and steel plates so that there is no longer any friction between them. When the clutch lever is pulled, the engine and transmission are two completely separate entities. Usually the coil springs clamp at around 200-300 pounds per square inch, which is more than enough to prevent the clutch from slipping. The clamping of coil spring feels positive on the rider's hand, and if there is a shock load to the transmission the clutch slips rather than breaking gears, sprockets or chains. For example, a shock load could come from a miscalculated jump.
Motorcycle clutches require the generation of sufficient centrifugal force to clamp the clutch plates together without substantial slippage and without utilizing a larger sized case which exceeds the space allowances available in most motorcycles. In the past, a pressure control mechanism was devised which combines the features of an automatic clutch with the performance of a traditional manual clutch so that the clutch engages smoothly without the use of a clutch lever at low speeds but limits the actual force transmitted to the clutch plates by the cam members at higher speeds. At the same time, the pressure control mechanism cooperates with a manual override lever to minimize the hand pressure required to override the cam members and effectively operate as a conventional manual clutch with relatively light feel or manual pressure and yet be capable of operating within the same space limitations as the standard or stock motorcycle clutches. Further, with hydraulic actuated clutches, there is no way of adjusting the hydraulic fluid in order to act on a clutch pack.
The prior art discloses various clutch mechanisms for motorcycles. One example is Youngwerth, Jr. et al., U.S. Patent Publication Number 2009/0242351, for an Automatic Clutch employing Expanding Friction Disk And An Adjustable Pressure Plate, which discloses an expanding friction disk assembly that expands under centrifugal force to provide automatic engagement of the clutch.
Yet another example is Maimone, U.S. Pat. No. 6,533,056 for a Motorcycle Automatic Clutch With Manual Release which discloses the use of a release plate which is movable between a first and second position to manually disengage the clutch.
Yet another example is Youngwerth, U.S. Pat. No. 6,957,730 for a Clutch Apparatus With An Automatic Centrifugal Engagement Of Pressure Plate which discloses a centrifugal pressure plate that moves axially which causes the plates to couple thereby forcing engagement between the engine and the transmission.
There is still a need for a mechanism to easily convert a standard motorcycle clutch into a centrifugal clutch.